Update build instructions / coding conventions.

This improves the instructions for checking out LLDB, adds
more links to LLVM instructions, and more explicitly calls out
when we differ from the style guide.

Also updates the clang-format configuration file to correctly
reflect a 120 column limit.

Reviewed by: Jim Ingham
Differential Revision: http://reviews.llvm.org/D8222

llvm-svn: 231884